This is the first step in determining effects of carbon nanotubes in the environment.

Green’s idea was to expose the sample to low-power microwaves and measure the resulting
increase in temperature.

Carbon nanotubes (CNTs) could pave the way for remarkable technology, from improved
computer chips, flexible computer screens or body armor, to health applications such
as bone healing and cancer treatments.

In an effort to determine the toxicity of these materials, a group of Texas Tech
researchers has successfully built a testing apparatus that can quantify the presence
of CNT in a given sample. It is a process easier said than done.

CNTs explained

A CNT is a carbon allotrope, just like graphite, charcoal, diamonds or graphene –
the world’s strongest known substance.

To the layman, graphene is a super-thin sheet of carbon atoms arranged in a hexagonal
“honeycomb” pattern. Conventional pencil graphite is simply many layers of graphene
stacked together. When rolled into a tube, graphene forms a CNT, a fiber 100 times
stronger than steel and six times lighter.

“What makes nanotubes even more remarkable is the fact that so many of their properties
are off the charts,” said Micah Green, assistant professor of chemical engineering.
“They are both electrically and thermally conductive, plus they are mechanically
strong. It is rare that a substance would combine all three.”

Industrial uses are growing, as are concerns that these novel nanomaterials may have
negative or unintended effects on organisms and the environment. With this in mind,
environmental toxicologists at Texas Tech are exploring the fate of CNTs in biological
environments and their ability to accumulate in soil, plants or other organisms.

One recurring question has slowed these studies: How can anyone be certain the tiny
CNTs are present in the given sample?

“It’s like a needle in a haystack,” Green said. “How can you prove the effects of
the needle, if you’re not sure that it’s really in there?”

Cooking up a solution

The impetus for the work initially began with a conversation between Green and Jaclyn
Cañas, associate professor of environmental toxicology at The Institute for Environmental and Human Health at Texas Tech. Cañas described the problem of detecting CNTs in crop samples. Green
suggested that exposing samples to microwaves could reveal the presence of even trace
quantities of nanotubes.

Experimental procedure for preparing root samples with known CNT quantities for generation
of calibration curve. Click to enlarge.

CNTs have the unusual property of evolving extreme amounts of heat upon exposure
to microwaves, much more so than typical materials. In fact, nanotube powder will
quickly and spontaneously ignite if placed in a conventional kitchen microwave. Green’s
idea was to expose the sample to low-power microwaves and measure the resulting increase
in temperature.

Mohammad Saed, an associate professor in electrical and computer engineering, joined
the team to contribute his expertise in the area of microwave physics.

Together, the three research groups successfully built a testing apparatus and proved
the concepts that microwave-based heating can quantify CNT loading inside a plant
sample.

Further Testing

Continued development of the device led to a double-blind test, where a student was
given samples of a specified CNT loading but was not told what the concentration
was. Graduate student Fahmida Irin was principally responsible for applying the method.
The double-blind test successfully duplicated the true values, and was then applied
to studying the uptake of nanotubes into alfalfa plant roots grown in soil spiked
with nanotubes.

“Since we started the method, we have started collaborating with other groups as
well to look at the presence of nanotubes in organisms like earthworms,” Green said.